Method for producing printing images on printing plates



Patented Oct. 7, 1930 UNITED STATES PATENT OFFICE OTTHEINRICH STRECKER,OF DABMSTADT, GERMANY METHOD FOR PRODUCING PRINTING IMAGES ON PRINTINGPLATES No Drawing. AppIication fiIedFebruary 16, 1929, Sria11fo.840,658, and in Germany February 8, 19,28.

The difliculty in producing printing images by the photo-mechanicalmethod, the

so-called copying method, is due to the peculiarity of the printingimage carriers. At the present time printing is mostly effected by meansof zinc or aluminium plates. For copying purposes there are used in thisconnection almost exclusively colloid solutions (albumen, fish glue, LePage and so forth), rendered sensitive to light by bi-chromate, thetreatment of such colloid solutions being well known and it is thereforenot necessary to describe it separately. When developing it is alwaysfound that the colloids are retained by the metal with considerableforce everywhere and even at the unexposed points. Probably the metalhas a tanning action on the colloids and holds these firmly in thethinnest possible layer which is insoluble or 0 only soluble withdifliculty. It will be quite clear that this causes difiiculties.

When using the albumen copying method printing is effected from theexposed albumen particles. When there has been sufiicient exposure thealbumen adheres very firmly. Some of it, however, also adheres to theunexposed points and during the printing part- 1y collects ink wherebythe printed images become fuller. Attempts to remedy this defect byenergetic etchings or weaker exposure do not help in any way. In thelatter case the printed images at the most acquire a slighter hold.Further, the albumen bi-chromate so- 3 lutions which are still liquidare very sensitive and have become considerably worse after two to threedays.

Prepared and dried albumen layers vary considerably under the action ofheat and 40 storing after copying. If. for example, a

negative of a plate is copied a number of times in succession, this, asis well known,

appears in a. very disturbing manner. Above all a comparatively largeamount of albumen remains in the first copies and is baked between thescreen point and causes a disturbing appearance, in the mannerdescribed, later on when printed.

Whereas it is apparently not possible in the so-ealled albumen copyingmethod to eliminate the defects referred to, it will be shownhereinafter how the copying method with reversal, which at the presenttime is still unsatisfactory and in which the unexposed parts are usedfor the production of the printed image,.can be carried out. Thesemethods differ from the albumen copying methods less by the selection ofthe colloids than by the fact that by means of the copy a negative incolloid is first produced on the printing plate. The actual printingimage is quite sharp and apparently blank without a coating of colloidafter development.

Actually some colloid is still present on the apparently blank points asalready mentioned in the form of a very thin, discontinuous layer, thatis to say, in the form of a veil or gauze. For removing this colloidfilin 'a large number of methods are already ayailable. The film isdisturbed, for example, by a mechanical action with pumice stone flouror by chemical treatment with acids. In the latter case there is usedabout 2% acid in alcohe] or glycerine so that the actual copy will notbe disturbed atthe same time.

Recently it has been found that dilute nitric acid only slightly attacksthe actual copy layer so that the seemingly blank exposed parts have'the film removed therefrom by deep etching. By this method aconsiderable variation in the tone values is, however, always produced.Disregarding the fact that all these means either do not sufiicientlyremove the film or remove it uniforml and more or less damage the actualcolloid lmage when the action is long or energetic, or vary the tonevalues, this method finally has further difiiculties.

The printing plate, according to the known method, is provided'directlywith printing ink or firstly with an intermediate layer. Thereupon thecolloid copy, together with the parts of the layer which cover it, 1smostly washed withdilute hydrochloric acid so that the printing imageremains. Dllute acid, however, cannot. in any way completely remove thecolloid as by reason 0 exposure and the adhesive power of the metal itis combined extremely firmly with the metal. It may also be assumed thatwith granular printing plates the lowermost colloid plates have agreater adhesion. When printing toning points are nearly alwaysexhibited as in thecourse of time the colloid residues take up ink. Evenvery energetic etchings cannot ing to the present invention thedisturbing colloid film is removed by treatment with certain saltsolutions, having the common property of effecting a direct solution ofa thin layer of glue or gelatin from the surface. This is accomplishedby first washing off with water the prepared glue layer, after exposure,so that the unexposed portions of the layer are dissolved and removed.The colloid veil of course remains notwithstanding this dissolvingoperation. Thereunon the copy is thoroughly dried with the aid of coldair, so that any trace of colloid and the colloid veil may be removed inthe manner described without endangering the copy. that is the exposedlayer, on the entire expose-d plate.

Probably the action of such salt solutions consists, according to theirnature, in that the salts partly effect adissolving of the colloidsunder conditions without mechanical, chemical or thermal assistance, asfor example, potassium bromide or ammonium nitrate. It is, however, alsopossible to use salts which wlthout any such assistance do not dissolvecolloids, for example salts of fatty acids, iron sulphate or sodiumsulphate. Consequently the treatment by rubbing with a wadding pad orthe like is apparently also effected by reason of the fact th at by thefriction and slight softening of the uppermost layer of the colloidimage the film is loosened and removed mechanically. Finally all saltsolutions have a so-c l e e-acidifying action on the metal of theprinting plate so that-at these points, after removal of the film, theprinting ink and the like will adhere efficiently.

It is also possible to use a number of compounds together in onesolution, such as a mixture of a solution of a soluble alkaline salt ofa higher fatty acid which is inert to the material of the plate andwhich has the property of dissolving unexposed colloid with any one ofthe other salts named herein, that IS, potassium bromide, ammoniumnitrate, iron sulphate, sodium sulphate, sodium chloride. Further,different solutions may be used in succession, in which-case it sadvisable to effect an intermediate rin'sing and renewed drying. Thesolutions may be used cold or under circumstances also hot. Finally aportion of the water in the solu- "tions may also be replaced forexample by alcohol and/or glycerine.

Solutions of "about 10 to 30% strength can be used but frequently aconsiderably lower concentration is suflicient. As a rule those meansare preferred which allow the metal of the printing plate to appearbright, influ ence it favourably for the further treatment, and whichcan be allowed to act for an indefinitely long time, as for examplesalts of fatty acids or common salt, as when removing the film anyinjury to the colloid copy and any modificationof the tone values mustbe effectively avoided.

The fatty acid salts referred to may be, for example, the oleates,palmitates, stearates or butyrates of sodium, potassium, or ammonium.These salts are preferably those of the higher fatty acids, that is, theacids above acetic.

After the removal ofthe film the copy is copiously and energeticallyrinsed with Water and completed in the usual manner.

Of particular advantage and of great importance isthe combination of theremoval of the film above described with the completion of the treatmentby the use of strong alkalis. for example soda lye. After removing thefilm, rinsing and drying the plate which has been treated to this extentis provided with a uniformly'thin suitable coating capable of resistingthe action of alkali, for example of asphalt. After this there ispreferably effected a preliminary treatment with a very weak acid forexample in a vessel containing water weakly acidified with hydrocholricacid (about 0.2% H01) The plate is then rinsed, soda lye applied theretoand the colloid image, in so far as this is still present, removed byrubbing with a wadding pad or the like.

Contrary to the treatment with any acids or ammonia the strong alkalican actually remove the colloid without residue. The printing imagesconsequently remain com pletely clean on the support which is otherwisenot the case.

The process briefly summed up is as follows: The zinc plate is coatedwith a film of colloid made sensitive with bichromate. This is exposedto light, and is developed by being washed in water. The plate is then,dried and afterwards treated with some of the salts named above toremove the portions of the colloid which although unexposed, have notbeen completely removed by the water. The plate is then rinsed and driedand coated with asphalt. It is then developed witha weak acid andrinsed, and the remaining colloid and any asphalt which may liethereonis removed by rubbing with soda lye and a wadding pad.

It has been proposed hitherto, in connection with reversing copyingmethods, to'apply iron chloride to the copy developed with water andtherefore having a swollen exposed layer, for the purpose of etching theplate, or to add alum for removing the acid. Both substances, however,cannot by themselves, that is without mechanical aids, remove thecolloid veil completely, so that the effect of these salts used in awell known manner will not insuresutficiently the adherence of the inkto the metal. Iron chloride when used without rubbing will remove theveil only in ammonium nitrate, iron sul .fattyacid which is inert anindirect way, that is to say only when owing to itsacid content it willeat away the metal undervthe colloid veil.

Salts which attack the metal of the printing plate or affect this by theformation of unfavorable deposits should not be used, as for example inthe case of zinc plates, copper. bromide (CuBr and iron chloride (FeGl Iclaim 1. In the process for producing printed images on printing platesby photomechani- I cal methods, the steps of developing the exposedplate, and then removing the thin veil of colloidal material whichadheres to the unexposed portion of the plate after the firstdevelopment by treating the plate with a s0- lution of a solublealkaline salt of a higher fatty acid. Y

2. In a process for producing printed images on printing plates byphotomechanical methods, the steps of developing the exposed plate, andthen removing the thin veil of colloidal material which adheres to theunexposed portion of the plate after the first development by treatingthe plate with a solution of a salt of the group comprising potassiumbromide, ammonium nitrate, iron sulphate, sodium sulphate, sodiumchloride, and, the soluble alkaline salts of the higher fatty acids.

3. In the process for producing printed images on printing plates byphotomechanical methods, the steps of developing them:- posed plate, andthen removing the thin veil of colloidal material which adheres to theunexposed portion of the plate after the first development by treatingthe plate with a solution of a soluble alkaline salt of a higher fattyacid which is inert to the material of the plate and which has theroperty of dissolving the colloid, and an addition of a salt of thegroup comprising potassium bromide, phate, sodium sulphate, sodiumchloride. 4. In the process for producing printed images on printingplates by photomechanical methpds, the steps of deve oping the exposedplate, then removing the thin veil of colloidal material which adheresto the urn exposed portion of the plate after the first development bytreating the plate with a solution of a soluble alka i to the materialof the plate and which has the property of dissolving the colloid,hereafter coatin plate with a layer of asphalt, and final y removing theexposed colloid parts by a strong alkali.

\ In testimony whereof I have signed my name to this specification,

QTTHEIN RICH STRECKER.

ne salt of a higher the v

